This invention relates to a process for producing filled polyurethane elastomers, and more particularly to a process for producing highly filled polyurethane elastomers in which resin solid separation is substantially reduced.
Urethane polymers or polyurethanes are a large family of polymers with widely varying properties and uses. Urethane polymers are generally classified into two broad categories. The first is a polyurethane foam, and the second is a polyurethane elastomer. Polyurethane foams are polyurethane polymers produced by the reaction of polyisocyanates with a hydroxyl group from a polyol and a polymerization catalyst, in the presence of water and/or an auxiliary blowing agent. The blowing agent allows the polymeric mass to expand into a cellular mass upon reaction.
In preparing a non-cellular polyurethane elastomer, no blowing agent or mechanism for producing gas which would lead to cell development should be present. Therefore, in the later case, a non-cellular urethane polymer is produced by the reaction of the isocyanate with a hydroxyl group to form urethane linkages in the presences of a polymerization catalyst. Apparatuses for making polyurethane materials are known. Various apparatuses for metering polyurethane reactants and thereby forming the requisite polyurethane material are described in the following patents: U.S. Pat. No. 2,730,433; U.S. Pat. No. 3,012,977; U.S. Pat. No. 3,319,937; U.S. Pat. No. 3,377,297; U.S. Pat. No. 3,431,081; U.S. Pat. No. 3,927,162; U.S. Pat. No. 4,038,037; U.S. Pat. No. 5,182,313; U.S. Pat. No. 5,476,638; U.S. Pat. No. 5,578,655; and U.S. Pat. No. 5,604,267.
Non-cellular polyurethanes can be used, for example, in wood products applications, such as plywood patch applications, for filling void areas therein such as crevices, holes, and other product imperfections. Typically, these compositions are applied in an assembly line fashion, thereby requiring a rapid cure at room temperature (generally less than one minute). The uses contemplated herein for filling void areas in wood products include plywood substrates and railroad ties.
Conventional polyurethanes are made by reacting two-components, i.e., a polyol resin that already contains catalyst, and a polyisocyanate. A conventional two-component process mixes the catalyst component into the resin during its manufacture, prior to transportation and storage. When even a small amount of catalyst is added to the resin in the conventional two-component process, the viscosity of the polyol resin is substantially reduced.
When the uncured liquid reaction mixture is dispensed into plywood defects the presence of a lower viscosity polyurethane reduces the amount of overfill waste (i.e., polyurethane that has mounded above the height of the panel). The polyurethane overfill waste is then sanding off to create a smooth, defect free, plywood panel. It is estimated that polyurethane overfill waste represents half of the polyurethane used to repair plywood defects.
When filled polyol resin is manufactured in a centralized location, and transported hundreds of miles to plywood mills for the filling of plywood defects, filler separation problems occur since the separation rate of the particulate solids is significantly increased. In the wood products industry, polyol resin separation is a major detriment causing the polyurethane materials to excessively shrink or chip out.